Does anyone know if there is such a thing out there?
I don't wish to "upgrade" to 11 speed due to the £ and the only benefit I could see of moving from 10 speed + 42t expander is having a 10 tooth low gear.
Would a 10 tooth work on say an XT 10 speed rear mech?
Nope, the ten tooth doesn't fit a standard freehub body hence the xd required for sram 11 speed.
Mountain tamer used to make a cog/lockring before 11 tooth cogs but you had to respace the axle and reddish the wheel.
It's a smaller diameter than a freehub body....which makes attaching it problematical.
XX1 uses a special freehub body.
Capreo uses a special feehub body (canfield did a cassette to mate to a capreo hub for a bit)
Hope I suspect will use a special freehub body
(There is a theme here).
Leonardi do a 9-42t 10sp cassette but you'll need an xd driver (sram compatible)
Do you need the full 42 tooth low gear?
How about moving up to a 34 or 36 up front to gain the higher gear?
I've been running a 32 up front with an 11 - 36 casette on a number of bikes for nearly a year and I do miss the high gear for the road sections riding to the trails, or occasionally on fire roads. I'm tempted to get the one up 42 tooth expander so I can then go to a 34 or 36 up front. Will still give a lower gear than I've got currently, but also give a higher gear for the faster sections...
I think moulton's had a 9 or10tooth cog effectively on the locking to assist with the small wheels